A vinylidene fluoride/hexafluoropropylene copolymer, a tetrafluoroethylene/propylene copolymer, a tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer, etc. are known as fluoro rubbers.
Among them, the tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer is called a perfluoroelastomer, which is remarkably excellent in heat resistance and chemical resistance, and also in plasma resistance.
A known crosslinking method for a perfluoroelastomer is a method in which a perfluoroelastomer containing an iodine atom, a bromine atom or the like is crosslinked with a peroxide (e.g. Japanese Patent No. 3508136, for example). Among them, a perfluoroelastomer containing iodine is widely used because a crosslinking time is short and no metal oxide needs to be added for crosslinking.
A method for producing an iodine-containing perfluoroelastomer is a method of copolymerizing perfluoro monomers in the presence of a diiodo compound represented by the general formula RI2 (wherein R is a saturated fluorohydrocarbon group or chlorofluorohydrocarbon group having a carbon number of from 1 to 16, or a hydrocarbon group having a carbon number of from 1 to 3), to produce a perfluoroelastomer with an iodine atom at a polymer terminal (e.g. Japanese Patent No. 3508136, for example). However, when the iodine content in the perfluoroelastomer produced by the above method is adjusted to achieve a necessary crosslinking density as a rubber, the iodine content is so large that the crosslinking proceeds rapidly, to cause scorching. On the other hand, if the iodine content was decreased to an amount low enough to avoid scorching, there was a problem that the crosslinking density was too low to provide satisfactory rubber properties.
Furthermore, a rubber molded article obtained by crosslinking of the perfluoroelastomer produced by the above method had a disadvantage such as low tensile strength, as compared with molded articles of other fluoro rubbers such as the vinylidene fluoride/hexafluoropropylene copolymer and tetrafluoroethylene/propylene copolymer. Particularly, in a case where the perfluoroelastomer is used as a sealing material for semiconductor manufacturing equipment, there is a demand for minimizing a content of a blending material in molding of a rubber. For example, carbon black, which is widely and generally used as a reinforcing material for rubber, becomes a source of dust from the sealing material and then there was a demand for decrease in the blending amount thereof. However, the decrease in the blending amount of the reinforcing material of carbon black tends to raise a problem that the tensile strength of the perfluoroelastomer molded article is considerably decreased.
Therefore, there was a following method reported: an iodine content in a polymer terminal was lowered and a crosslinking density was controlled by copolymerizing iodine—or bromine-containing monomers as represented by CF2═CF-Rf1—X (wherein Rf1 is a saturated polyfluoroalkylene group or polyfluorooxyalkylene group and X is I or Br) (e.g. Japanese Patent No. 3508136 and Japanese Patent No. 2888972, for example).
When the crosslinking density was increased by this method, other important properties such as the elongation at break and compression set property were sometimes deteriorated.
There was no known perfluoroelastomer composition capable of facilitating a kneading operation and providing a perfluoro rubber molded article having a sufficiently high tensile strength, and an excellent elongation at break and compression set property.
It is known to add one of quinones and phenols as a scorch retarder in order to control a crosslinking reaction rate in a crosslinking step of elastomer, in cases of hydrocarbon rubbers, and fluorine-containing rubbers except for perfluoro rubbers (e.g. JP-A-5-271478 and JP-A-10-101879, for example).
However, it is not known to add a scorch retarder in the peroxide crosslinking of the perfluoroelastomer, and it is totally unknown to achieve an effect of improvement in physical properties of the resulting molded article, in addition to the prevention of scorching.
Under these circumstances, there are strong desires for a perfluoroelastomer composition good in processability, unlikely to scorch, and capable of providing a perfluoro rubber molded article through crosslinking with excellent rubber properties, such as the tensile strength and elongation at break, and with an excellent compression set property.